I'm using a full synthetic. With what I've got into the tractor good lubricants and good filters are a small price to pay. I do shop for the best prices on my choice of oil, though.
Always have used synthetic oil. With it, you don't need to change yearly like you would dino oil irregardless of hours. Run synthetic to the full 200 hours. Will provide better protection of top end on cold starts, particularly if you use 0w, 5w or 10w 40 wt oil or 0w or 5w or 10w 30 wt and not the very thick 15w40.
Though I am a fan of synthetics in any of my cars (especially the ones that require it), it's always been non synthetic Rotella 15w 40 in any of my marine and equipment diesels. That engine will outlast me with Rotella in it and regular oil changes. If I used the tractor in the winter, I'd consider going with 10 w 30, though.
A youtube video on the top 5 synthetic oils listed Rotella T6 5w40 diesel oil as one of them.
I like M1 0w30. It is European diesel rated. In both the Kubota B and Isuzu generator driver. For the Isuzu, it will be good for the 6 to 10 years until I change it again unless we get A LOT of power failures.
I’ve always used the T6 in my LS and in my Yanmar powered SxS. Still do my oil changed yearly just because oil/filters are cheap (relatively) and engines are not. Really not experiencing the life benefit of synthetic but get all the others.